The use of proteolytic enzymes as condensation catalysts for the stereospecific coupling of two L-amino acids to yield L, L-peptides is known since the early days of protein chemistry. As early as 1909 Mohr and Strohschein described the formation of the water-insoluble dipeptide Bz-Leu-Leu-NH.sub.2 by reacting Bz-Leu-OH and H-Leu-NH.sub.2 in the presence of the protein degrading enzyme papain. E. Mohr and F. Strohschein, Ber 42, 2521 (1909). The Mohr and Strohschein-type reaction is possible only between those amino acids that form peptide bonds that are susceptible to cleavage by the papain or other enzyme used. Indeed, the condensing reaction's equilibrium between the amino acid reactants and peptide product is largely displaced towards the reacting amino acids. Nevertheless, the condensing reactant can be driven to completion by mass action if, e.g., the dipeptide product is poorly soluble and precipitates out of the reaction phase.
Due to the commercial importance of certain peptides and the fact that enzymes are known to catalyze peptide formation under mild conditions there has been a great deal of research done on the enzymatic synthesis of peptides particularly simple dipeptides. K. Oyama and K. Kihara, Kagaku Sosetsu 35, 195 (1982); K. Oyama and K. Kihara, ChemTech. 14, 100 (1984).
The process for enzymatic synthesis of the peptide derivative aspartame, described in U.S. Pat. No. 4,165,311, hereinafter the '311 process, involves the thermolysin-catalyzed condensation of N-carbobenzoxy-L-aspartic acid with D,L-phenylalanine methyl ester and precipitation of an intermediary complex, D-phenylalanine methyl ester salt of N-carbobenzoxy-aspartame, to drive the reaction to the peptide product side. Further processing of this intermediary complex allows for the recovery of D-phenylalanine methyl ester, that may be recycled after racemization, and of the N-carbobenzoxy-aspartame derivative which can be converted to aspartame by elimination of the N-carbobenzoxy protecting group. The '311 process must be practiced on a batch basis which is cumbersome and complicates the recovery of enzyme. Also see: K. Oyama, S. Irino, T. Harada and N. Hagi, Ann. N.Y. Acad. Sci. 434, 95 (1985).
The N-carbobenzoxy protecting group plays an essential role in the '311 process by: fulfilling the structural requirement imposed by the active site of thermolysin; and by contributing to the insolubility of the intermediary complex thereby increasing the yield of the reaction. Elimination of the N-carbobenzoxy protecting group from the aspartame derivative must be effected under mild conditions, e.g., catalytic hydrogenation, to prevent cleavage of the methyl ester function. Catalytic hydrogenation involves the inconvenience of handling hydrogen gas on a large scale.
Alternative approaches to driving enzymatic condensation reactions to completion have also been described in the chemical literature. For example, the use of organic solvents as reaction media has been found effective for increasing the peptide product yields; although, the concomittant decrease in enzyme stability has precluded its practice on a large scale. K. Oyama, S. Nishimura, Y. Nonaka, K. Kihara and T. Hashimoto, J. Org. Chem. 46, 5241 (1981); H. Ooshima, H. Mori and Y. Harano, Biotechnology Letters 7, 789 (1985); K. Nakanishi, T. Kamikubo and R. Matsuno, Biotechnology 3, 459 (1985).
In view of the above-noted difficulties in the practice of prior art methods for enzymatic synthesis of peptides, particularly, the requirements for precipitation of an intermediary complex and handling of dangerous reagents; it would be desirable to provide an improved process that avoids these difficulties and that safely provides effective yields without rapid deactivation of the enzyme catalysts.